Automatic threshold compensating demand pacemaker

ABSTRACT

In a demand cardiac pacer having means for providing periodic stimulating pulses to the heart, means for sensing natural heartbeats, and means for inhibiting the stimulating pulses on the occurrence of natural heartbeats, the improved apparatus including means for sensing stimulated heartbeats, means for decreasing the amplitude of a succeeding stimulating pulse on the occurrence of a stimulated heartbeat and means for increasing the next succeeding stimulating pulse by a predetermined amount on the failure to sense a stimulated heartbeat.

United States Patent 11 1 Mulier et al.

1 51 Sept. 11, 1973 AUTOMATIC THRESHOLD COMPENSATING DEMAND PACEMAKER [75] inventors: Pieter M. J. Mulier, Minneapolis;

John R. Helland, Brooklyn Center, both of Minn..

3,523,539 8/l970 Lavezzo l28/4l9 P 3,618,615 11/1971 Greatbatch l28/4l9 P 3,638,656 2/1972 Grandean et al 128/419 P Primary Examiner-William E. Kamm Attorney-Lew Schwartz [57] ABSTRACT In a demand cardiac pacer having means for providing periodic stimulating pulses to the heart, means for sensing natural heartbeats, and means for inhibiting the stimulating pulses on the occurrence of natural heartbeats, the improved apparatus including means for sensing stimulated heartbeats, means for decreasing the amplitude of a succeeding stimulating pulse on the occurrence of a stimulated heartbeat and means for increasing the next succeeding stimulating pulse by a predetermined amount on the failure to sense a stimulated heartbeat.

28 Claims, 6 Drawing Figures 52 U.S. c1. 128/419 P, 128/422, 307/264, 328/169, 331/183 511 1111. c1 A6ln 1/36 [58] Field 61 Search 128/419 P, 421, 422; 331/183; 328/169; 307/264 [56] 1 References Cited UNITED STATES PATENTS 3,669,120 6/1972 Nielsen 128/419 P 3,426,748 2/1969 Bowers 128/419 P SENSE STIMULATION PULSE AMPLIFIER '1 GENERATOR 12 11 15 PATEMEUSEPY W 1973 3 u 7 57 792 Arman/5y -|.O ms STIMULATING PULSE CARDIAC DEPOLARIZATION RESPONSE MAXIMUM OUTPUT VO LTS PATIENT THRESHOLD I TIME 4 INVENTORS Pie ter, M (Ill [alien BY John B. lfelland PATENTEU Y 3.75T. T92

SHEET 3 IF 3 MAXIMUM OUTPUT VOLTS PATIENT THRESHOLD 0 TIME {PRIOR ART OUTPUT VOLTAGE RANGE OUTPUT AUTOMATIC THRESHOLD VOLTAGE COMPENSATING DEVICE (VOLTS) OUTPUT VOLTAGE RANGE TIME (SECONDS) INVENTORS :J IG E Pieter l LcZ/l [ulien John B. A eZlanoZ AUTOMATIC THRESHOLD COMPENSATING DEMAND PACEMAKER BACKGROUND OF THE INVENTION Demand cardiac pacers, usually R-wave responsive, are well-known in the art of electronic cardiac treatment. Such devices sense natural heartbeats to inhibit stimulating pulses to the heart in the presence of natural beats, and provide periodic stimulating pulses to the heart in the absence of natural beats. The sensing amplifiers of such devices generally have a refractory period of sufficient length to mask the initial responses of the heart to the stimulation pulses, that is, the stimulated heartbeats. The refractory period is necessary due to artifacts caused by polarization of the electrodes which act both as stimulating and sensing electrodes. A typical one of such demand type pacers is Medtronic, Inc. No. 5842.

Such prior art demand devices inherently carry high safety margins. The stimulation energy is set at an amount known to be several times that of the maximum required threshold energy in most patients. As the threshold energy of the patient is not constant, and may vary during a day, and as the threshold information is not known to the device, maximum energy is always provided for stimulation. Obviously, this results in maximum drain from the power sources, and the result is shortened lifetime for the cardiac demand device. It is to overcome this problem that the present invention has been made, and the present invention has been found to operate with significantly less battery drain than the prior art demand devices.

It has been long known in the art that information related to the patients cardiac stimulus threshold is useful for many purposes, such as detecting end of life for a pacer, and various prior art schemes have existed for sensing threshold levels. One such prior art threshold device is that described in The Proceeding of the Nordic Meeting on Medical and Biological Engineering, An Experimental Pacemaker: A Method For Automatic Determination of Heart Stimulation Threshold, by J. Meiborn and LS. Nielsen, Jan. 15, 18, I970, Finland. in this article there is described a pacer which in the pacing mode provides that each stimulating impulse is decreased by a given amount after each detection of a stimulated heartbeat. Sensing of the stimulated heartbeat occurs for 100 milliseconds following each pacing impulse, and the pacing impulses are continually reduced until no stimulating heartbeat is sensed during the 100 millisecond period. When no stimulating heartbeat is sensed, a new pacing impulse of full amplitude is generated immediately after the 100 millisecond period, and the reduction process commences again. Because the pacing impulses are reduced in amplitude by a predetermined amount commencing with full value, the threshold level can be determined by noting at which pulse heart capture is lost. To avoid the problem of electrode polarization and the resulting refractory period, this device uses separate pairs of electrodes for stimulating and sensing.

Though the prior art cardiac device described in the preceding paragraph teaches detection of threshold levels, it does not teach and has little effect on battery drain as the stimulating pulse returns to full amplitude following each loss of capture. The apparatus of this invention provides for greatly decreased battery drain and thus greatly increased device lifetime by sensing each stimulated heartbeat and providing that each succeeding stimulating pulse be decreased in energy until such time as no stimulated heartbeat occurs. When loss of capture is sensed, the next succeeding stimulating pulse is increased in energy, but is increased by an amount only enough to be safely over the threshold hysteresis level and not to maximum level. In one embodiment of the present invention, an early extra stimulating pulse of maximum energy is provided following loss of capture, and the following normal pulse is reduced to a level only sufficiently above the threshold hysteresis level to be safe, and it is from this reduced pulse that further stimulating pulse energy reductions occur until capture is once again lost.

SUMMARY OF THE INVENTION Briefly described, the apparatus of this invention comprises a demand cardiac pacer, R-wave responsive, with no built-in refractory period. Sensing of natural and stimulated heartbeats are accomplished with a single sensing amplifier. Separate sensing and stimulating electrodes are preferable, each working with a third common electrode, preferably with a large enough conductive area to avoid significant polarization problems. The output stimulation energy is provided from a capacitor which is connected across a battery by time constant means and a switch. Each output pulse will decrease the charge on the capacitor so that the succeeding output pulse will be of less energy. Should an output pulse occur and no stimulated heartbeat be sensed by the amplifier, a second timed pulse circuit is provided, in addition to the output pulse circuit, which will activate the switch for a predetermined period of time to allow charging of the capacitor prior to the next output stimulation pulse. Thus more energy will be available for the output pulse following an output pulse which results in loss of capture. The period of time for which the switch is on, and the time constant means connected to the capacitor, are predetermined such that the energy level of the capacitor will increase an amount sufficient to provide a stimulation pulse safely above the threshold level, taking into account threshold hysteresis, but not maximum level unless it is needed. In one embodiment of the invention, to avoid loss of a heartbeat due to loss of capture, an immediate stimulation pulse of maximum energy level is provided, and the next output pulse comes at the level below maximum value and just safely above threshold level. In that embodiment, the maximum level stimulation pulse is provided following a short sensing period of approximately milliseconds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a combination block diagram and schematic of the first embodiment of the apparatus of this invention;

FIG. 2 is a schematic diagram of a portion of FIG. 1, showing components added to make a second embodiment of the apparatus of this invention;

FIG. 3 is a graph of a generalized stimulated heartbeat response using the apparatus of this invention;

FIG. 4 is a stylized graph of the output pulses during the pacing mode of the apparatus of FIG. 1;

FIG. 5 is a stylized graph of the output pulses of the apparatus of FIG. 2; and

FIG. 6 is a graph of output voltage verses time for a prior art'demand pacer, and for the apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the figures of the drawings, it should be understood that the graphs of FIGS. 3 6 are intended to be representative of the plots they disclose, and are not necessarily drawn to scale. It should also be understood that the term stimulating pulse as used herein means an output pulse from the apparatus of this invention, whether or not the heart responds to the pulse. Also, as used herein, both the natural occurring and artificially stimulated cardiac depolarizations are described and shown as QRS signals.

Referring first to FIG. 1, there is disclosed a sense amplifier 11 shown in block diagram. Amplifier 11 is of the type generally used in demand pacers and wellknown to those familiar with the art, and includes such circuits as extraneous noise or interference signal filters, but excludes a designed long refractory period and preferably has a design to prevent saturation.

There is also shown in FIG. 1 a stimulation pulse generator 15, again shown in block diagram as such generators are wellknown to those skilled in the art. Generator includes timing means for providing periodic signal pulses, which timing means are reset when the signal pulse is provided or when a signal from sense amplifier 11 indicates that an R-wave has been sensed from the heart.

Sense amplifier 11 has an input connected to a sensing electrode 12 adapted to be connected to a portion of the heart for sensing heartbeats. The output of sense amplifier 11 is connected to the input of generator 15 and to a reset bus 14. The output of generator 15 is connected through a serial combination of a diode 17 and resistor 18 to the base of an output transistor 20. The collector of transistor 20 is connected through a resistor 21 to a positive bus 22 adapted to be connected to the positive terminal of a source of energy such as batteries. The collector of transistor 20 is also connected through an output capacitor 23 to a stimulating electrode 25 adapted to be connected to a portion of the heart for providing stimulating pulses during the pacing mode of the apparatus of this invention. An electrode 26 is connected to positive bus 22. Electrode 26 acts as a common electrode for both of electrodes 12 and 25 and is preferably of comparatively large dimension to avoid polarization effects.

The emitter of transistor 20 is connected to a junction 27. The base of transistor 20 is connected through a resistor 28 to junction 27. Junction 27 is connected to bus 22 through a capacitor 29. Junction 27 is connected through an inductor 30 to the collector of a transistor 31. The emitter of transistor 31 is connected to a negative bus 32, adapted to be connected to the negative terminal of the source of energy. The base of transistor 31 is connected through a resistor 33 to bus 32 and through a resistor 39 to a junction 34. Junction 34 is connected through a resistor 35 to the collector of a transistor 36. The emitter of transistor 36 is connected to positive bus 22. The collector of transistor 36 is connected through a serial combination of diode 37 and a resistor 38 to reset bus 14. The base of transistor 36 is connected through a resistor 41 to bus 22, and is directly connected to the collector of a transistor 42.

The emitter of transistor 42 is connected through a resistor 43 to negative bus 32. The base of transistor 42 is connected through a capacitor 45 to junction 34. The base of transistor 42 is also connected through a resistor 46 to a junction 47. Junction 47 is connected through a capacitor 48 to bus 32.

A transistor 50 has its collector connected to junction 47, and its emitter is connected through a resistor 51 to bus 32. The collector of transistor 50 is also connected through a resistor 52 to the collector of a transistor 55. Transistor 55 has its emitter connected to bus 22. The base of transistor 55 is connected through a resistor 56 to bus 22, and is connected through a diode 57 to bus 22. The base of transistor 55 is also connected through a capacitor 58 to a junction 61. Junetion 61 is connected through a resistor 62 to bus 32. Junction 61 is connected through a serial combination of a diode 63 and a resistor 64 to the output of generator 15. The base of transistor 50 is connected to reset bus 14, and is connected through a serial combination of a diode 67 and a resistor 66 to the output of generator 15.

The modes of operation of the apparatus of this invention as shown in the preferred embodiment of FIG. 1 are similar to those of prior art demand cardiac pacers. The apparatus has a standby mode when the heart is beating naturally, thus causing the reset of generator 15 following each sensed natural heartbeat to prevent an output pulse for stimulating the heart. There is also a pacing mode which is initiated when no natural heartbeat is sensed and no reset of generator 15 occurs. The output signal pulse from generator 15 will then cause stimulation pulses to be felt between electrodes 25 and 26, in a unique manner to be more fully described below. Finally, the apparatus of FIG. 1 is constructed in a manner known to those skilled in the art such that on failure of the sensing mechanism no reset pulses will reach generator 15 and the entire apparatus will operate as an asynchronous pacer with maximum energy stimulation.

In the apparatus of this invention as shown in the embodiment of FIG. 1, and the same sense amplifier 11 which is used in prior art demand pacers is also used to control a second pulse generating circuit comprising, in brief, transistors 36 and 42 along with timing capacitor 48 and reset transistor 50. In a manner more fully described below, if a stimulated heartbeat is not sensed between electrodes 12 and 26 for a predetermined period of time following a stimulation signal to the heart, the second pulse generator will provide a signal to transistor 31 causing output voltage capacitor 29 to be charged for a period of time determined by capacitor 45. Thus, the next stimulation pulse will be at a greater energy magnitude than the preceeding stimulation pulse. Should a stimulated heartbeat be detected by electrodes 12 and 26 during the predetermined time,

then a signal on reset bus 14 will turn on transistor 50 to discharge capacitor 48. There will be no output from the second pulse generator, and transistor 31 will remain off. Thus, capacitor 29 will not be charged and the next stimulation pulse will be ofa decreased energy magnitude by an amount equal to the energy expended from capacitor 29 in the previous stimulation signal.

To best understand the operation of the apparatus of FIG. I, assume first that the device has been newly connected to the heart and that capacitor 29 is at maximum charge. Assume now that natural heartbeat fails,

putting the apparatus of FIG. 1 into a pacing mode. Stimulation pulse generator will then provide an output signal pulse approximately I millisecond in duration. This pulse will have the several effects described below.

First, the stimulation pulse generator 15 output signal pulse will pass through diode 17 and resistor 18 to turn on transistor 20. The full charge across capacitor 29 will be felt across resistor 21 and thus across electrodes 25 and 26 to stimulate the heart to which electrode 25 is connected. This stimulation signal will be at maximum energy due to the maximum charge on capacitor 29.

The output signal pulse from generator 15 will also be felt through resistor 66 and diode 67 to turn on reset transistor 50, thus discharging capacitor 48, the timing capacitor for the second pulse generator. At the same time, the output signal pulse from generator 15 will cause the discharge of timing capacitor 58. This discharge is through diode 57 and bus 22 through a portion of generator 15 (not shown) and back through resistor 64 and diode 63.

After the l-millisecond output signal pulse from generator 15, capacitor 58 will commence to recharge through resistors 56 and 62. The recharge time for capacitor 58 is selected to provide the window during which the apparatus of FIG. 1 will sense the hearts response to the stimulation pulse between the electrodes 25 and 26. In the preferred embodiments of FIG. 1, this time is selected to be 100 milliseconds. During the charge time of capacitor 58, transistor 55 will be turned on, and at the end of 100 milliseconds transistor 55 will turn off.

The turn-on of transistor 55 completes a charge path for capacitor 48, comprising bus 22, transistor 55, resistor 52, capacitor 48 and bus 32. This charge time is the period for the second pulse generator and is selected to be lessthan the window time provided by capacitor 58, which in this preferred embodiment it is selected to be 80 milliseconds. The reasons for the, timing selections will be more fully described below with reference to FIG. 3 of the drawings.

If the charge of capacitor 48 is allowed to continue for 80 milliseconds, the resulting voltage level will turn on transistor 42, which will in turn switch on transistor 36 to provide an output pulse of a width determined by capacitor 45. This output pulse, felt at the collector of transistor 36' will pass through resistors 35, 39 and 33 to turn on transistor 31. The turn-on of transistor 31 enables the charging of capacitor 29 through the timing constant path comprising bus 22, capacitor 29, inductor 30, transistor 31 and bus 32. The timing constant for the charge of capacitor 29 is selected so that the recharge of capacitor 29 provides more energy than was expended in providing the last stimulation pulse. Of course, when the last stimulation pulse has been at maximum energy, capacitor 29 will only recharge back to maximum energy.

The output pulse on the collector transistor 36 will also pass through diode 37 and resistor 38 to turn on reset transistor 50 and discharge capacitor 48 thus turning off transistors 42 and 36 after the output pulse width determined by capacitor 45.

It is to be noted that the above operation of the apparatus of FIG. 1 describes the method of recharging capacitor 29, which recharge occurs only when capacitor 48 is allowed to charge fora full predetermined period of time, in this case milliseconds during a window of milliseconds. The pulse from transistor 36 which actuates the time constant charging path for capacitor 29 will hereinafter be referred to as the escape pulse. It is desired to have the escape pulse provided only when the heart has not responded, during the window time, to the stimulation pulse provided between electrodes 25 and 26, that is, only when the energy provided by the stimulation pulse is below the stimulation threshold of the patient to cause cardiac response.

If a stimulated cardiac response is sensed by amplifier 11 across electrodes 12 and 26, amplifier ill will provide an output pulse, which, in addition to resetting the timing of generator 15, will be felt on reset bus 14 to turn on transistor 50. The turn-on of transistor 50 will discharge capacitor 48 thus restarting the 80 millisecond charge time for the second pulse generator. If the signal on bus 14 has occurred during the first 20 milliseconds of the window time, capacitor 48 will still have sufficient time to charge through transistor 55 to a level sufficient to turn on the second pulse generator to provide the escape pulse. However, should the sensing of the stimulated heartbeat occur after the first 20 milliseconds of the window time, capacitor 58 will reach a full charge and turn off transistor 55 to prevent the charge on capacitor 48 from reaching a turn-on level for the escape pulse. Failure to produce the escape pulse will result in no recharge of capacitor 29, and the next stimulating output pulse will be provided at an energy level below that of the preceding pulse. Further, the next output pulse signal from generator 15 will reset capacitor 48, as described above, so that the watch time for the escape pulse generation will commence with capacitor 48 starting uncharged.

The cycle of operation described in the above paragraph, where no escape pulse is provided and the stimulation energy provided from capacitor 29 decreases with each succeeding stimulating signal by an amount equal to the energy expended in the preceding stimulating signal, will continue causing a train of cardiac stimulation pulses of decreasing energy until such time as the stimulation energy falls below the stimulus threshold of the patients heart. When this occurs and capture is lost, no sensing of stimulated heartbeat is felt at amplifier ill and capacitor 48 is not reset during the win dow period. Then, as described above, an escape pulse will be provided from transistor 36 and transistor 31 will turn on to charge capacitor 29. However, the charge on capacitor 29 will not. return to maximum level in all cases, but only to an amount greater than that last stimulation pulse which did provide capture. The amount of charge of capacitor 29, when an escape pulse is provided, is determined by the on time of the escape pulse which is determined by capacitor 45 as described above, and by the timing constant means including inductor 30. Capacitor 45 and inductor 30 are selected to assure that there is a sufficient recharge of capacitor 29 to provide a safety margin, recognizing that there is a stimulus threshold hysteresis effect in most patients, and that the stimulation pulse following the pulse which loses capture must be sufficiently high to provide capture on the next stimulation pulse.

When capture has been lost and an escape pulse has provided a recharge of capacitor 29, as described above, the cycling of the apparatus will continue and if the first stimulation pulse following recharge of capaci tor 29 is sufficient to cause a stimulated heartbeat, the

next and succeeding stimulation pulses will each be reduced in energy until capture is again lost and another escape pulse is provided. Thus, the apparatus of this in vention as shown in the preferred embodiment of FIG. 1 will seek the patient threshold level and will operate with stimulation energy sufficient only to provide the minimum necessary safety margins. By providing that the charge on capacitor 29 has not returned to maximum energy level upon loss of capture, the apparatus of this invention achieves great savings in power supply drain, and thus adds significant time to the life of the automatic threshold compensating demand pacer of this invention.

The apparatus of this invention is intended to be used primarily as a fully implantable device. Therefore, the entire apparatus as shown in FIG. 1, along with batteries used as a source of energy, is intended to be encased in a substance substantially inert to body. fluids and tissue and placed subcutaneously in the body. Electrodes 12 and 25 are preferrably connected to the heart to perform their respective sensing and stimulating functions. Common electrode 26 may, for example, be an external metal enclosure of the implanted device, but is in any case preferrably of a large conductive surface to avoid depolarization problems.

Reference to FIG. 3 will enable a better understanding of the selection of the window timing for the above described apparatus of FIG. 1. The cardiac depolarization response, or the QRS complex, of the heartbeat is known to those familiar with the art. FIG. 3 shows a 1- millisecond stimulating pulse from the apparatus of FIG. 1, followed by the QRS response from the heart. It can be seen that the QRS response occurs within approximately milliseconds after the stimulating pulse, and that the R-wave occurs within approximately 75 milliseconds after the stimulating pulse. The peak of the R-wave occurs approximately 50 milliseconds after the stimulating pulse. It can also be seen that the T- wave does not occur until approximately 210 milliseconds after the stimulating pulse and lasts for approximately 100 milliseconds. It is the QRS complex that the apparatus of FIG. 1 wishes to sense to determine whether or not to provide an escape pulse. As is known to those familiar with the art, the timing of the waves shown in FIG. 3 will vary from patient to patient, and may vary within the patient himself. It has been experimentally shown, and is generally accepted by those familiar with the art, that the R-wave will be completed in virtually all patients within 100 milliseconds after the stimulating pulse. It is for this reason that the window has been chosen at that length for the preferred embodiment. It is also generally known from experiments that stimulation artifacts may occur during the first 10 to milliseconds after the stimulating pulse. Because such an artifact may cause a reset pulse to appear from amplifier 11 on reset bus 14 to reset capacitor 48, the timing of the escape pulse of the apparatus of FIG. 1 has been set for 80 milliseconds. Thus, should a reset due to an artifact occur during the first 20 milliseconds and the actual R-wave not be stimulated, the apparatus of FIG. 1 will still be able to provide an escape pulse to recharge capacitor 29. Should the artifact cause resetting during the first portion of the window, and the R- wave in fact occur during the window, then the escape pulse will be blocked as described above.

The window feature described in the discussion of FIG. 1 is a desirable but not a mandatory feature. The

apparatus of FIG. I could work reliably simply by providing a refractory period of approximately 20 milliseconds in amplifier 11 to eliminate artifacts, and by directly sensing the R-wave and providing an escape pulse should the R-wave not occur in the timing period of an escape pulse generator. However, in a second embodiment shown in FIG. 2 and to be fully described below, the window becomes mandatory to avoid providing a stimulating pulse during the hearts vulnerable period.

The second preferred embodiment of the apparatus of this invention is best understood with reference to FIG. 2. The second embodiment of this invention includes all of the apparatus of the embodiment of FIG. 1 with the addition of one electrical component and one electrical interconnection. In FIG. 2 that portion of the circuitry of FIG. 1 including these additional connections is shown, and it will be understood that the apparatus shown in schematic form in FIG. 2 is connected to the circuit the same as that of FIG. 1. The same descriptive numbers are used for like members of the circuit with new numbers indicating only the additions.

Referring to FIG. 2, there is again shown output ener gy capacitor 29, inductor 30, and transistor 31, serially connected between buses 22 and 32. A diode is added in serial connection between inductor 30 and transistor 31. There is also shown electrodes 25 and 26 connected across resistor 21 by circuitry including output capacitor 23. Transistor 20 is again shown connected between resistor 21 and junction 27. An added electrical conductor 81 is shown connecting the collector of transistor 20 to the collector of transistor 31.

The purpose of the second embodiment is to provide an early maximum energy stimulating pulse to the heart when the preceding stimulating pulse has resulted in loss of capture. It will be recognized that in the embodiment of FIG. 1, as the stimulating pulses step down to an energy level below the patients threshold, one heartbeat will be missed by the patient before the next increased energy stimulating pulse due to the charging of capacitor 29 caused by an escape pulse. It is to avoid this occasional missed heartbeat that the second embodiment may be used.

The operation of the circuitry of FIG. 2 is precisely the same as that of FIG. 1 with the single exception that when an escape pulse does result from the second pulse generator, the turn-on of transistor 31 will not only commence charging of capacitor 29, but will also, through conductor 81, connect electrode 25 through capacitor 23 and transistor 31 to bus 32. Thus electrodes 25 and 26 will be directly across buses 22 and 32 and the maximum energy available from the power supply will be applied directly to the heart. This application, due to the window feature of the escape pulse, will occur within milliseconds after loss of capture is sensed. Therefore, the stimulating output pulse which fell below the patients threshold level will be followed very quickly by a maximum level output pulse to provide the missed stimulation. Also, because the escape pulse performs its normal function of causing the charging of capacitor 29, the next normal stimulating pulse following the maximum energy pulse will be less than maximum but at a level above threshold in exactly the same manner of operation as that described above with regard to the operation of FIG. 1.

In reviewing the operation of the embodiment of FIG. 2, it will become apparent why, as mentioned above, the window is a mandatory feature. As previously described, it is known that the R-wave it is desired to sense will occur within 100 milliseconds after the stimulating pulse. As sensing of the R-wave is directly followed by a maximum energy stimulating pulse in the second embodiment, it is important to assure that that maximum energy stimulating pulse does not occur during the patients vulnerable time period following the QRS complex. This vulnerable period is wellknown to those familiar with the art, and could result in the patient entering fibrillation. Therefore, the win dow is provided in the embodiment of FIG. 2 to assure that the sensing of artifacts or, for example, the T- wave, will not cause the output of a maximum energy stimulating pulse following the 100 millisecond period in which the R-wave normally occurs.

The operating principles of the embodiment of FIGS. 1 and 2 can be more fully understood with references to FIGS. 4 and 5. FIG. 4 is a stylized graph of volts versus time in which the vertical lines represent the voltage levels of the stimulating pulse outputs from the embodiment of FIG. 1, assuming that the output energy capacitor 29 is charged to maximum at the beginning of operation, as was assumed in the above discussion of the operation of FIG. 1. The top horizontal line of FIG. 4 indicates the maximum output voltage available, the level at which the first stimulating pulse occurs. The lower horizontal line in the graph of FIG. 4 indicates the patient threshold level. It will be recognized to those familiar with the art, that though respresented as a single line on the graph, the patient's threshold level is instead a wider band of voltage range, due to a hysteresis effect found in many patients whereby once stimulation has dropped below the stimulus threshold level of the patient, to regain capture it is necessary to raise the stimulus voltage to a point above that level at which capture was lost.

From FIG. 4 it is apparent that each succeeding output stimulating pulse is decreased in amplitude, until the level of the stimulating pulse falls below the level of the patients threshold. At that time capture is lost and an escape pulse causes the charging of output capacitor 29. The next succeeding pulse is therefore raised to an energy level greater than the last pulse which resulted in capture, but less than maximum pulse level. Thereafter the process is repeated until the stimulating pulse level falls below threshold again, whereafter the next stimulation pulse is again increased and the process repeated yet again. It is to be carefully noted that the stimulus threshold of cardiac response in a patient may vary considerably, and may in fact vary in a single patient during the day. The apparatus of this invention will always seek and follow the threshold level whether it rises or falls. The graph of FIG. 4 indicates how the stimulating pulses seek the threshold level and then ride that level to provide a safe margin of stimulus at a minimum energy expenditure. Finally, though the graph of FIG. 4 is in terms of volts versus time, it will be recognized that the apparatus of this invention could work equally as well by varying other pa rameters of the output stimulating pulse, with the important factor being the amount of stimulating energy supplied to the heart.

With reference to FIG. 5, there is shown exactly the same graph as in FIG. 4, but as applied to the second embodiment of this invention of FIG. 2. It will be noted that the operation of the embodiment of FIG. 2, as

shown in FIG. 5, is exactly the same as that of the first embodiment, as shown in FIG. 4, with the exception that shortly following each loss of capture, one maximum energy pulse is provided to the heart to avoid loss of a heartbeat. Thereafter, the succeeding stimulating pulse from the apparatus of FIG. 2. is the same battery drain-saving, low level as that of the first embodiment, from which the cycle will repeat again.

As stated above, the advantage of the second embodiment of this invention over the first embodiment is the avoidance of the loss of a heartbeat in the patient. However, the second embodiment has a disadvantage with respect to the first embodiment in that it does use somewhat more energy. When the apparatus of the first and second embodiments are in storage and not connected to a patient, as they are completely selfcontained battery operated units adapted for implantation, each will be outputing maximum energy pulses, however, the second embodiment will be outputing extra pulses of maximum energy, that is, twice as many pulses as the first embodiment and thus causing significantly greater battery drain during storage. This same double pulse effect will occur when embodiments are implanted, should sensing be lost.

FIG. 6 is a graph on a scale of output voltage versus time, of both the prior art demand pacer output voltage level, and the output voltage level of the apparatus of this invention as shown in the embodiment of FIG. 1. From FIG. 6 it becomes clear that the output voltage required by the apparatus of FIG. 1 is significantly less than the prior art output voltage throughout the entire operation of the device. Though the threshold level of the patient may vary, seen from FIG. 6, the prior art demand cardiac pacers always provides maximum energy, while the apparatus of the present invention seeks the threshold level and thereafter provides the minimum safe energy necessary to achieve cardiac stimulation.

A further feature of the apparatus of this invention, found in both the first and second preferred embodiments, is the ability to remember the energy level at which the last stimulation pulse occurred. That is, should the automatic threshold compensating demand pacer of this invention leave the pacing mode and enter the standby mode due to the sensing of natural heart beats, capacitor 29 will hold the charge remaining after the cycle following the last stimulated heartbeat. When the apparatus of this invention again enters the pacing mode due to failure to sense a natural heartbeat, the next normal stimulating pulse will be at the energy level held by capacitor 29, as though the pacing mode had not been interrupted. This feature of the apparatus of this invention obviously adds significantly to the decrease in battery drain. If the heart of the patient should respond intermittently with natural heartbeats, the battery drain would be greatly increased if the pacer returned to maximum stimulation following each natural heartbeat.

It will be apparent that the principles embodied in the above described preferred embodiments of this invention can be encompassed in structures other than those specifically shown, without departing from the scope of the invention.

We claim:

ll. Cardiac pacer apparatus having first means for providing timed cardiac stimulation pulses, second means for sensing natural heartbeats, means connecting the second means to the first means for inhibiting ill stimulation pulses when a natural heartbeat is sensed, and electrode means connected to the first and second means and adapted to be connected to a heart, the apparatus including: third means connected to the electrode means for sensing stimulated heartbeats; fourth means connected to the first and third means and responsive to each sensing of a stimulated heartbeat for decreasing the energy of the succeeding stimulation pulse by a predetermined amount, the fourth means responsive to each failure to sense a stimulated heartbeat for increasing the energy of the succeeding stimulation pulse to an amount greater than the preceding stimulation pulse but normally less than maximum energy; and

energy source means connected to all the means.

2. The apparatus of claim 1 including; further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.

3. The apparatus of claim 2 in which: the further means provides the extra stimulation pulse at the maximum energy level.

4-. The apparatus of claim 2 in which the electrode means comprises: stimulating electrode means connected to the first means and adapted to be connected to a heart; sensing electrode means connected to the second means and adapted to be connected to a heart; and further electrode means electrically common to the first and second means and the stimulating and sensing electrode means, and adapted to be connected to a portion of the body.

5. The apparatus of claim 4 in which: the further electrode means includes dimensioned conductive surface means for minimizing polarization effects of stimulation pulses.

6. The apparatus of claim il in which the second and third means comprise: a single sense amplifier means having input means connected to the electrode means and output means connected to the first and fourth means.

7. The apparatus of claim 6 including: further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.

8. The apparatus of claim 7 in which: the further means provides the extra stimulation pulse at the maximum energy level.

9. The apparatus of claim 1. including: fifth means connecting the first means to the fourth means for inhibiting operation of the fourth means in the absence of a stimulation pulse.

10. The apparatus of claim 9 including: further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.

11. The apparatus of claim 10 in which: the further means provides the extra stimulation pulse at the maximum energy level.

12. The apparatus of claim 10 in which the fifth means includes: means responsive to a stimulation pulse for enabling operation of the fourth means for a limited time period following each stimulation pulse.

13. The apparatus of claim 9 in which the fifth means includes: means responsive to a stimulation pulse for enabling operation of the fourth means for a limited time period following each stimulation pulse.

14. The apparatus of claim 1 in which the electrode means comprises: stimulating electrode means connected to the first means and adapted to be connected to a heart; sensing electrode means connected to the second means and adapted to be connected to a heart; and further electrode means electrically common to the first and second means and the stimulating and sensing electrode means, and adapted to be connected to a portion of the body.

15. The apparatus of claim 14 in which: the further electrode means includes dimensioned conductive surface means for minimizing polarization effects of stimulation pulses.

16. An implantable demand cardiac pacer comprising: sense means for sensing electrical signals from a heart; first timed pulse generator means; energy storage means; controllable output means connected across the energy storage means; controllable energy charge path means connected to the energy storage means; second timed pulse generator means; stimulation electrode means connected to the output means and adapted to be connected to a heart; sense electrode means connected to the sense means and adapted to be connected to a heart; means connecting the sense means to the first and second timed pulse generator means for resetting the first and second generator means on the sensing of an electrical signal from the heart; means connecting the first generator means to the output means for controlling stimulation output pulses; means connecting the second generator means to the controllable energy charge path means; the the charge path means including means responsive to the second generator means for controlling charging of the energy storage means such that the energy level to which the storage means is charged is proportional to the energy in the storage means following the last stimulation output pulse; and all the means adapted to be connected to a source of energy.

17. The apparatus of claim 16 in which: the energy storage means comprises a capacitor; and the charge path means includes time constant means connected in series with the capacitor.

18. The apparatus of claim 17 in which the time constant means includes inductance means.

19. The apparatus of claim 16 including: inhibit means connected between the first generator means and the second generator means; the inhibit means including means normally operative for inhibiting operation of the second generator means and means responsive to a pulse from the first generator means for allowing operation of the second generator means for a predetermined time period.

20. The apparatus of claim 19 in which: the second timed pulse generator has a pulse repetition rate less than the predetermined time period.

21. The apparatus of claim 19 including; further means connecting the first generator means to the second generator means for resetting the second generator means in response to a pulse from the first generator means.

22. The apparatus of claim 19 including: means connected between the output means and the second generator means and responsive to a pulse from the second generator means for connecting the stimulation electrode means across the source of energy.

23. The apparatus of claim 16 in which: the sense means includes amplifier means having a refractory period not greater than 20 milliseconds.

24. In an implantable cardiac demand pacer having a source of energy, sensing means connected to the source of energy, periodic pulse generator means con nected to the source of energy, and means connecting the sensing means to the generator means for resetting the generator means, the improvement comprising: sense electrode means connected to the sensing means and adapted to be connected to a heart; capacitor means; time constant means; first switch means; means connecting the time constant means, the first switch means and the capacitor means in series across the source of energy; output means including stimulation electrode means adapted to be connected to a heart; second switch means; means connecting the output means and the second switch means in series across the capacitor means; means connecting the pulse generator means to the second switch means; second periodic pulse generator means connected to the source of en ergy; means connecting the sensing means to the second pulse generator means for resetting the second pulse generator means; and means connecting the second pulse generator means to the first switch means for controlling connection of the capacitor means and the time constant means to the source of energy.

25. The apparatus of claim 24 in which: the time constant means includes inductance means.

26. The apparatus of claim 24 including: third switch means connected between the second pulse generator means and the source of energy for normally inhibiting operation of the second pulse generator means; window timing means connected between the periodic pulse generator and the third switch means for allowing operation of the second pulse generator means for a limited window period of time following a pulse output from the periodic pulse generator.

27. The apparatus of claim 26 in which: the periodic pulses from the second generator are at a repetition rate less than the window period of time.

28. The apparatus of claim 26 including: means connected between the first switch means and the output means for connecting the output means across the source of energy when the first switch means is actuated. 

1. Cardiac pacer apparatus having first means for providing timed cardiac stimulation pulses, second means for sensing natural heartbeats, means connecting the second means to the first means for inhibiting stimulation pulses when a natural heartbeat is sensed, and electrode means connected to the first and second means and adapted to be connected to a heart, the apparatus including: third means connected to the electrode means for sensing stimulated heartbeats; fourth means connected to the first and third means and responsive to each sensing of a stimulated heartbeat for decreasing the energy of the succeeding stimulation pulse by a predetermined amount, the fourth means responsive to each failure to sense a stimulated heartbeat for increasing the energy of the succeeding stimulation pulse to an amount greater than the preceding stimulation pulse but normally less than maximum energy; and energy source means connected to all the means.
 2. The apparatus of claim 1 including; further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.
 3. The apparatus of claim 2 in which: the further means provides the extra stimulation pulse at the maximum energy level.
 4. The apparatus of claim 2 in which the electrode means comprises: stimulating electrode means connected to the first means and adapted to be connected to a heart; sensing electrode means connected to the second means and adapted to be connected to a heart; and further electrode means electrically common to the first and second means and the stimulating and sensing electrode means, and adapted to be connected to a portion of the body.
 5. The apparatus of claim 4 in which: the further electrode means includes dimensioned conductive surface means for minimizing polarization effects of stimulation pulses.
 6. The apparatus of claim 1 in which the second and third means comprise: a single sense amplifier means having input means connected to the electrode means and output means connected to the first and fourth means.
 7. The apparatus of claim 6 including: further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.
 8. The apparatus of claim 7 in which: the further means provides the extra stimulation pulse at the maximum energy level.
 9. The apparatus of claim 1 including: fifth means connecting the first means to the fourth means for inhibiting operation of the fourth means in the absence of a stimulation pulse.
 10. The apparatus of claim 9 including: further means connected to the first and third means and responsive to each failure to sense a stimulated heartbeat for providing an extra stimulation pulse prior to the normal succeeding stimulation pulse.
 11. The apparatus of claim 10 in which: the further means provides the extra stimulation pulse at the maximum energy level.
 12. The apparatus of claim 10 in which the fifth means includes: means responsive to a stimulation pulse for enabling operation of the fourth means for a limited time period following each stimulation pulse.
 13. The apparatus of claim 9 in which the fifth means includes: means responsive to a stimulation pulse for enabling operation of the fourth means for a limited time period following each stimulation pulse.
 14. The apparatus of claim 1 in which the electrode means comprises: stimulating electrode means connected to the first means and adapted to be connected to a heart; sensing electrode means connected to the second means and adapted to be connected to a heart; and further electrode means eLectrically common to the first and second means and the stimulating and sensing electrode means, and adapted to be connected to a portion of the body.
 15. The apparatus of claim 14 in which: the further electrode means includes dimensioned conductive surface means for minimizing polarization effects of stimulation pulses.
 16. An implantable demand cardiac pacer comprising: sense means for sensing electrical signals from a heart; first timed pulse generator means; energy storage means; controllable output means connected across the energy storage means; controllable energy charge path means connected to the energy storage means; second timed pulse generator means; stimulation electrode means connected to the output means and adapted to be connected to a heart; sense electrode means connected to the sense means and adapted to be connected to a heart; means connecting the sense means to the first and second timed pulse generator means for resetting the first and second generator means on the sensing of an electrical signal from the heart; means connecting the first generator means to the output means for controlling stimulation output pulses; means connecting the second generator means to the controllable energy charge path means; the the charge path means including means responsive to the second generator means for controlling charging of the energy storage means such that the energy level to which the storage means is charged is proportional to the energy in the storage means following the last stimulation output pulse; and all the means adapted to be connected to a source of energy.
 17. The apparatus of claim 16 in which: the energy storage means comprises a capacitor; and the charge path means includes time constant means connected in series with the capacitor.
 18. The apparatus of claim 17 in which the time constant means includes inductance means.
 19. The apparatus of claim 16 including: inhibit means connected between the first generator means and the second generator means; the inhibit means including means normally operative for inhibiting operation of the second generator means and means responsive to a pulse from the first generator means for allowing operation of the second generator means for a predetermined time period.
 20. The apparatus of claim 19 in which: the second timed pulse generator has a pulse repetition rate less than the predetermined time period.
 21. The apparatus of claim 19 including; further means connecting the first generator means to the second generator means for resetting the second generator means in response to a pulse from the first generator means.
 22. The apparatus of claim 19 including: means connected between the output means and the second generator means and responsive to a pulse from the second generator means for connecting the stimulation electrode means across the source of energy.
 23. The apparatus of claim 16 in which: the sense means includes amplifier means having a refractory period not greater than 20 milliseconds.
 24. In an implantable cardiac demand pacer having a source of energy, sensing means connected to the source of energy, periodic pulse generator means connected to the source of energy, and means connecting the sensing means to the generator means for resetting the generator means, the improvement comprising: sense electrode means connected to the sensing means and adapted to be connected to a heart; capacitor means; time constant means; first switch means; means connecting the time constant means, the first switch means and the capacitor means in series across the source of energy; output means including stimulation electrode means adapted to be connected to a heart; second switch means; means connecting the output means and the second switch means in series across the capacitor means; means connecting the pulse generator means to the second switch means; second periodic pulse generator means connected to the source of energy; means connecting the sensing means to the second pulse genErator means for resetting the second pulse generator means; and means connecting the second pulse generator means to the first switch means for controlling connection of the capacitor means and the time constant means to the source of energy.
 25. The apparatus of claim 24 in which: the time constant means includes inductance means.
 26. The apparatus of claIm 24 including: third switch means connected between the second pulse generator means and the source of energy for normally inhibiting operation of the second pulse generator means; window timing means connected between the periodic pulse generator and the third switch means for allowing operation of the second pulse generator means for a limited window period of time following a pulse output from the periodic pulse generator.
 27. The apparatus of claim 26 in which: the periodic pulses from the second generator are at a repetition rate less than the window period of time.
 28. The apparatus of claim 26 including: means connected between the first switch means and the output means for connecting the output means across the source of energy when the first switch means is actuated. 